1. Field of the Invention
The present invention relates to a hybrid working machine using both engine power and electric power.
2. Description of the Related Art
The related art will be described taking an excavator which is a preferred example of the present invention as an example.
The excavator is, as shown in FIG. 3, formed by rotatably mounting an upper rotating body 2 on a crawler type lower traveling body 1 and attaching a working attachment 3 to the upper rotating body 2.
The working attachment 3 is formed of a boom 4 capable of raising and lowering, an arm 5 attached to an edge of the boom 4, a bucket 6 attached to an edge of the arm 5, and a boom cylinder 7, an arm cylinder 8 and a bucket cylinder 9 which are hydraulic actuators for driving the boom, the arm and the bucket.
As a rotation actuator for rotating the upper rotating body 2, a hydraulic motor and a rotation motor serving as an electric actuator are used in the case of a hydraulic excavator and in the case of a hybrid excavator respectively.
The hybrid excavator having a driving mode of so called a parallel type is known (refer to Japanese Patent Laid-Open No. 2001-12274).
In this type, a hydraulic pump and a generator motor for performing a generator operation and a motor operation are connected in parallel to an engine serving as a common power source, and a hydraulic actuator is driven by the hydraulic pump. Meanwhile, a rotation motor is driven by the generator operation of the generator motor or an electric storage device.
The electric storage device is electrically charged by the generator operation of the generator motor. The generator motor timely performs the motor operation by discharge power of the electric storage device so as to assist the engine.
For example, in a machine in which engine speed sensing (ESS) control is performed, in correspondence with reduction in rotation speed of the engine, the assist control for controlling assisting force is performed in addition to power reducing control of the pump.
Irrespective of the ESS control, at the time of high load imposed, that is when only the engine is not enough to impose the load, there is a case where a shortage thereof is supplemented by an assisting action.
As mentioned above, in the hybrid excavator in which the generator motor performs the engine assisting action, the assist control is performed without any conditions other than assist conditions such as the rotation speed of the engine and the load. Therefore, even in the case where the assisting action is not required (and wasteful), that is in the case where the engine is stopped by overload or the like (engine failure) and in the case where the engine would clearly be stopped (including running out of fuel), the assisting action is still continued.
In this case, since wasteful consumption of stored power and over discharge are caused, it is not only unpreferable in view of saving energy, but also to decrease the life of the electric storage device.